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Re: Possible problem when two clients update the same record?

From: "Sean Davis" <sdavis2(at)mail(dot)nih(dot)gov>
To: "Karsten Hoffrath" <maillists(at)khoffrath(dot)de>,<pgsql-novice(at)postgresql(dot)org>
Subject: Re: Possible problem when two clients update the same record?
Date: 2005-07-23 14:36:42
Message-ID: 000801c58f93$f1a9f8e0$6401a8c0@WATSON (view raw or flat)
Thread:
Lists: pgsql-novice
----- Original Message ----- 
From: "Karsten Hoffrath" <maillists(at)khoffrath(dot)de>
To: <pgsql-novice(at)postgresql(dot)org>
Sent: Saturday, July 23, 2005 10:27 AM
Subject: [NOVICE] Possible problem when two clients update the same record?


> Hi all.
>
> I'm converting a dos based application using DBase files to PostgresSQL.
> The application is a pawn shop system.
>
> The dos application works as follows:
> Table A holds all items which the customers bring to the shop.
> Table B has one record for each day which holds the totals (pieces,
> costs, etc.) for this particular day.
> When B is updated, the particular record gets locked, updated and 
> released.
>
> The new application works this way:
> - start a new transaction,
> - insert the new record in A,
> - the insert operation fires a trigger which updates B
> - the transaction is committed.
>
> Now my question:
> What happens if two clients insert a new record to A at the same time?
> Will B get updated correctly or will one value get lost?

I can't see a reason why one value would get lost.  There will be two 
transactions, each independent of the other.  I'm not sure about what would 
lead to a deadlock in this case, but I would think that would be pretty 
unlikely, as the insert operation and update to B will take milliseconds at 
most.  B is not locked during the entire transaction, as you describe it 
above, I think.

Sean



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